Conventionally, the exhaust gas of a car contains pollutants such as unburned hydrocarbon (HC), an NOx gas and an SOx gas. In order to reduce the pollutants, therefore, Sulfur (S) in a gasoline or light oil is removed for the Sox or unburned Hydrocarbon (HC) is burned by a catalyst, for example.
More specifically, as shown in FIG. 13, a car system 100 takes air in through an automatic element (filter) 102 and feeds the air into an engine 106 through an airflow sensor 104. Moreover, the car system 100 feeds a fuel in a fuel tank 108 into the engine 106 through a fuel pump 110.
Based on the result of the detection of an A/F sensor 112, the injection of the fuel in the engine 106 is controlled by a fuel injection control device 114 in order to have a predetermined theoretical air fuel ratio.
For an exhaust gas fed from the engine 106, hydrocarbon (HC) in the exhaust gas is burned by a catalytic device 116 and is then discharged as the exhaust gas through an oxygen concentration sensor 118.
In consideration of the influence of the NOx in the exhaust gas on an environment, recently, there has been proposed a method of supplying a urea solution to the catalytic device 116, thereby reducing the NOx to be non-toxic as an N2 gas in order to decrease the NOx in the exhaust gas discharged from the fuel of a car, for example, a gasoline or light oil.
More specifically, as shown in FIG. 13, the car system 100 has such a structure as to supply a urea solution to the upstream side of the catalytic device 116 through a urea solution supplying mechanism 130 constituted by a urea solution tank 132 for storing a urea solution, a urea pump 134 and a urea spraying device 136 for spraying the urea solution fed from the urea pump 134 onto the upstream side of the catalytic device 116.
In such a car system, it is suitable that 32.5% by weight of urea and 67.5% by weight of H2O should be set in order to efficiently generate a reducing reaction at the upstream side of the catalytic device 116 without causing the urea solution to cake, for example.
For this reason, conventionally, NOx sensors 140 and 142 have been provided at the upstream and downstream sides of the catalytic device 116 respectively to measure the concentration of NOx in order to decide whether or not the concentration of a urea sprayed onto the upstream side of the catalytic device 116 is constant.
However, the NOx sensors 140 and 142 measure the concentration of the urea as a result of the reducing rate of NOx. For this reason, it is impossible to previously identify the concentration of the urea which is contained in the urea solution tank 132 or is sprayed. Moreover, the NOx sensors 140 and 142 do not have very high sensitivities.
The present inventors have already proposed a fluid identifying method in Japanese Laid-Open Patent Publication No. 11-153561 (particularly see paragraphs [0042] to [0049]). This method is to cause a heating member to generate heat by carrying electricity, heating a temperature detector through the heat generation, thermally influencing a heat transfer from the heating member to the temperature detector through a fluid to be identified, and distinguishing the type of the identified fluid based on an electrical output corresponding to the electric resistance of the temperature detector, thereby periodically carrying the electricity to the heating member.
In the fluid identifying method, however, it is necessary to periodically carry the electricity to the heating member (in a multipulse). For this reason, a long time is required for the identification so that it is hard to identify a fluid instantaneously. In this method, moreover, it is possible to identify a fluid based on a central value for substances having very different properties such as water, air and oil. However, it is hard to identify the concentration of the urea of the urea solution accurately and immediately.
In consideration of such circumstances, it is an object of the present invention to provide an apparatus and method for identifying the concentration of the urea of a urea solution which can identify the concentration of the urea of the urea solution accurately and immediately.
Moreover, it is an object of the present invention to provide an apparatus and method for reducing the exhaust gas of a car using the apparatus and method for identifying the concentration of the urea of a urea solution which can efficiently reduce the exhaust gas and can enhance a mileage.